Because of limited water supplies and the widespread use of irrigation, the soils of many cultivated areas have become increasingly salinized. In particular, modern agricultural practices such as irrigation impart increasing salt concentrations when the available irrigation water evaporates and leaves previously dissolved salts behind. As a result, the development of salt tolerant cultivars of agronomically important crops has become important in many parts of the world.
Dissolved salts in the soil increase the osmotic pressure of the solution in the soil and tend to decrease the rate at which water from the soil will enter the roots. If the solution in the soil becomes too saturated with dissolved salts, the water may actually be withdrawn from the plant roots. Thus the plants slowly starve though the supply of water and dissolved nutrients may be more than ample. Also, elements such as sodium are known to be toxic to plants when they are taken up by the plants.
Salt tolerant plants can facilitate use of marginal areas for crop production, or allow a wider range of sources of irrigation water. Traditional plant breeding methods have, thus far, not yielded substantial improvements in salt tolerance and growth of crop plants. In addition, such methods require long term selection and testing before new cultivars can be identified.
Genetic engineering and other methods have been used in attempts to improve crop plants by understanding the genetic basis for salt tolerance. For instance, considerable effort has been directed to the selection of salt tolerant plant cells and callus in vitro (see, e.g., Dix The Plant Journal 3:309-313 (1993)). A major barrier in the improvement of salt tolerance in crops is the poor understanding of the specific genes that have the potential of increasing salt tolerance (reviewed in Serrano et al., Crit. Rev. Plant Sci. 13:121-138 (1994)).
Genes associated with adaptation to salt stress have been identified in yeast. Serrano and coworkers have identified two genes, HAL1 (Gaxiola et al. EMBO J. 11:3157-3164 (1992)) and HAL2 (Glaser et al. EMBO J. 12:3105-3110 (1993)) in Saccharomyces cerevisiae by selecting for genes whose overexpression leads to improved growth on salt. A HAL1 homolog is present in plants where it is induced by NaCl and abscisic acid, a plant hormone known to mediate adaptation of plants to osmotic stress Murguia et al., Science 267:232-234 (1995)).
Another gene, calcineurin, or phosphoprotein phosphatase type 2B (PP2B), is a calmodulin-regulated enzyme found in many organisms including yeast. Although its physiological functions are not well understood, it is known that yeast strains which do not contain active calcineurin proteins are more sensitive to growth inhibition by salt than are wild-type strains. Bacterial genes associated with salt tolerance have also been identified. Tarczynski et al., Science 259:508-510 (1993)).
Despite the efforts toward cloning genes conferring tolerance to saline conditions, no single salt tolerance plant gene has been identified. The present invention addresses these and other needs.